Abstract

Proterozoic Al2SiO5 “triple-point” metamorphic rocks of north-central New Mexico are examined to honor the career contributions of Lincoln Hollister to petrology and tectonics, and to promote discussion of outstanding problems in metamorphic petrology. Hollister’s career began with studies of compositional zoning in garnet and staurolite, and interpreting the occurrence of coexisting Al2SiO5 polymorphs in British Columbia. These studies emphasized the kinetic and bulk composition controls of metamorphism. Hollister’s interest in the Al2SiO5 polymorphs led to his graduate student, Professor Jeff Grambling’s pioneering work that proposed the equilibrium occurrence of coexisting kyanite, sillimanite, and andalusite in the Truchas Peaks, New Mexico. Subsequently, polymetamorphism and the disequilibrium coexistence of kyanite, sillimanite, and andalusite have been proposed as alternate explanations for the occurrence of the polymorphs across the region.

Field stops in the Picuris Mountains and Tusas Mountains will visit classic metamorphic rock localities, which are the subject of debate regarding the equilibrium/disequilibrium nature of the regional “triple-point” metamorphism. The trip will examine Al2SiO5-bearing mineral assemblages to demonstrate the regional distribution of the polymorphs. Stops in the Picuris Mountains will examine andalusite ± cordierite, andalusite ± chloritoid, and kyanite + sillimanite + andalusite–bearing rocks. Field stops in the Tusas Mountains will show kyanite and sillimanite ± garnet–bearing assemblages. Garnet + biotite ± staurolite–bearing rocks are common in both areas and will be examined. Results of garnet, monazite, and zircon geochronology and their bearing on the P-T-t-D paths for the region and the timing of orogenesis will be discussed.

Introduction

This field guide, and accompanying topical session were organized to honor 50 years of career contributions by Lincoln Hollister to metamorphic petrology and tectonics. Fifty years ago, Hollister (1966) provided the first description and quantitative model for zoning of garnet, which became a classic paper in metamorphic petrology. The field trip will promote discussion of mineral zoning, the interpretation of mineral reaction textures, the role of reaction overstepping and the metastable persistence of minerals, and the application of thermodynamic databases and construction of equilibrium phase diagrams in metamorphic petrology. A careful review of the P-T figures referenced in this paper will show that the triple point of Holdaway (1971) or Pat-tison (1992) or both have been used by different studies. We have not attempted to reconcile these differences and will leave it as a point of discussion for the field trip.

Field-trip stops will visit bedrock exposures in the Picuris and Tusas Mountains to highlight specific mineral assemblages related to the overall regional distribution of the Al2SiO5 polymorphs across northern New Mexico (Figs. 1 and 2). Rocks exposed in these mountain ranges are part of a 150-km-long belt of aluminous quartzite and schist that we define as the regional metamorphic “triple-point” terrain of northern New Mexico. We use this terminology to refer to the areas where rocks with kyanite + sillimanite + andalusite occur and show no obvious influence of contact metamorphism. Our definition is much more geographically restricted than the 75,000 km2 triple-point terrane of Grambling et al. (1989). Previous work on the metamorphic rocks in the region will be briefly discussed, but this guide is not intended to be an exhaustive review of prior work.

Lincoln Hollister’s first paper was a description of compositional zoning in garnet and a quantitative model for fractional crystallization that explained the observed compositional profile (Hollister, 1966). His model did not depend on the assumption of chemical equilibrium, but rather the concentration of an element and its partition coefficients among coexisting minerals. This model is generally accepted as an explanation of growth zoning in garnet. It has recently been applied to explain trace element zoning profiles in garnet in the context of garnet geochronology (Kelly et al., 2011). It has also been fundamental for understanding the distribution of Lu and Hf isotopes used to determine garnet ages in sillimanite gneiss, which we will visit at Cerro Colorado in the southern Tusas Mountains (Aronoff et al., 2016).

Hollister followed up his paper on garnet zoning with a detailed study of compositional zoning in staurolite (Hollister and Bence, 1967). In this paper, the authors recognized that stau-rolite zoning corresponded with crystallographic sectors. Hollis-ter and Bence (1967) concluded that the composition of the different staurolite sectors is kinetically controlled and, thus, does not represent equilibrium mineral compositions. Both the garnet and staurolite papers emphasized kinetic and concentration controls on mineral composition. Hollister cemented his approach to understanding disequilibrium petrology with a key paper on contact metamorphism in the Kwoiek area of British Columbia (Hollister, 1969), the location of Hollister’s Ph.D. thesis study. In his 1969 paper, he argued that, during contact metamorphism, the association of kyanite + andalusite + sillimanite was the result of metastable growth of andalusite, and possibly fibrolite, rather than an equilibrium distribution across a field gradient.

Hollister’s work on sector zoning would not stay restricted to Earth, as he readily applied his knowledge of zoning in minerals to lunar samples collected during the Apollo missions. Hargraves et al. (1970) and Hollister and Hargraves (1970) described compositional sector zoning in lunar pyroxene from coarse-grained basalts. The compositional variations in the pyroxene were interpreted to result from rapid crystallization at the Moon’s surface, not differentiation in the lunar interior as previously inferred.

Following these early successes and landmark papers, Hol-lister returned to British Columbia to spend a significant portion of his research career on understanding granulite-facies rocks within the Coast Mountains of British Columbia. These rocks were among the first examples of regional granulite-facies meta-morphism discovered outside of Archean terranes (Hollister, 1975). Hollister applied new tools to understanding metamorphism, including the studies of fluid inclusions and geometric and kinematic analysis of metamorphic rocks. Hollister used these new tools to better understand the geology of British Columbia, and he published a series of seminal papers that highlighted his evolving thinking on the interactions between deformation, metamorphism, and melting in the deep crust.

These papers include Hollister (1979) and Hollister (1982), which described a clockwise P-T path to explain the evolution of granulite-facies rocks in the Coast Mountains. In these studies, he used mineral textures, which represent incomplete reactions, along with mineral compositions and fluid inclusions to constrain the P-T evolution of the rocks. In these two papers, Hollister also argued that uplift rates inferred from the metamorphic history are similar to what would be expected for erosion of thickened crust, one of the first examples of linking deep crustal evolution to surface processes in an active tectonic setting.

During the 1990s and early 2000s, Hollister led the ACCRETE Continental Dynamics project, which studied the formation of the Coast Mountains batholith of British Columbia. This work focused on the interplay of magmatism, convergence, transcurrent displacement, and large terrane transport in crustal formation. The interdisciplinary nature of the ACCRETE project involved integrating geophysics into the study of deep crustal formation (Morozov et al., 1998, 2001, 2003; Hollister et al., 2008). Hollister’s contributions to understanding the interactions of deformation and plutonism led to new models for orogeny-parallel translation in the Coast Mountains (Hollister and Andro-nicos, 1997; Andronicos et al., 1999; Chardon et al., 1999) and novel approaches to understanding terrane transport by using paleomagnetic data (Hollister et al., 2004). This work led to the idea that crustal-scale transpression and transtension facilitates the construction of batholiths and, thus, contributes to the formation of continental crust in continental margin arcs (Morozov et al., 2003; Hollister and Andronicos, 2006).

During his 43 years at Princeton University, Hollister always engaged undergraduate and graduate students in his research. Hollister led nearly annual field trips to New Mexico. Since Grambling’s early work on the triple-point terrane of New Mexico, several new models were proposed for the evolution of these rocks that questioned if they represent an equilibrium distribution of the Al2SiO5 polymorphs across a field gradient, or if the minerals represent polymetamorphism associated with regional metamorphism and deformation both ca. 1.65 Ga and ca. 1.4 Ga (Williams et al., 1999; Karlstrom et al., 2004). Hollister’s classes visited key localities across the triple-point terrane, including many of the stops that are the focus of this trip. Students on the trips collected samples and analyzed them for class projects, and a few of these turned into senior thesis projects. Barnhart et al. (2012) summarized the results of some of these studies and documented high-temperature decompression in sillimanite-grade rocks of the southern Tusas Mountains at Cerro Colorado. Most recently, Aronoff et al. (2016) used Lu-Hf garnet ages from these localities to understand the timing of Proterozoic orogenesis in the region. Thus, Hollister’s pioneering approach to understanding metamorphic rocks extends across 50 years and at least four generations of geologists.

Proterozoic Lithostratigraphy of Northern New Mexico

Bedrock exposures throughout northern New Mexico contain a wide array of ca. 1.75-1.45 Ga metamorphic rock with variable protoliths and bulk compositions. The oldest Paleoproterozoic successions are the Gold Hill, Pecos, and Moppin complexes (Bauer and Williams, 1989). These basement rocks are thought to have formed as a complex collage of predominately juvenile arc terranes that were accreted along the southern margin of the ca. 1.8 and 1.7 Ga Yavapai crustal province (Fig. 1) (Bow-ring and Karlstrom, 1990; Karlstrom et al., 2004; Whitmeyer and Karlstrom, 2007). These arc-related rocks are interpreted to be the “basement” to younger supracrustal successions.

Younger supracrustal successions exposed in the Picuris and Tusas Mountains (Figs. 2 and 3) include the 1.72 Ga Vadito Group, which is composed of mafic and felsic metavolcanic rocks, schist, and quartzite with minor pebble conglomerates (Bauer and Williams, 1989; Bauer, 1993). The Moppin complex and Vadito Group are crosscut by intrusions that range in age from ca. 1720 Ma to 1630 Ma (Karlstrom et al., 2004). In general, the Paleoproterozoic plutons are deformed and contain tectonic foliation of variable intensity. A single Mesoproterozoic pluton intrudes the Vadito Group in the southern Picuris Mountains. The Penasco granite (ca. 1450 Ma) is syn-to-post tectonic, with a narrow strain gradient at the margin and elliptical mafic enclaves within the interior of the pluton (Bauer, 1993).

Simplified lithostratigraphic section for the Tusas and Picuris Mountains. The ca. 1450 Ma Marqueñas Formation is only exposed south of the Plomo-Pecos fault and unconformably overlies the ca. 1710 Ma Vadito Group schist. This section is overturned in the southern Picuris Mountains. The ca. 1488 Ma or younger Trampas Group is exposed north of the Plomo-Pecos fault and continues along strike to the east in the Truchas Peaks and Rio Mora areas. The Rinconada Formation and overlying Trampas Group are not observed in the Tusas Mountains (Bauer and Williams, 1989; Williams, 1991).

Simplified lithostratigraphic section for the Tusas and Picuris Mountains. The ca. 1450 Ma Marqueñas Formation is only exposed south of the Plomo-Pecos fault and unconformably overlies the ca. 1710 Ma Vadito Group schist. This section is overturned in the southern Picuris Mountains. The ca. 1488 Ma or younger Trampas Group is exposed north of the Plomo-Pecos fault and continues along strike to the east in the Truchas Peaks and Rio Mora areas. The Rinconada Formation and overlying Trampas Group are not observed in the Tusas Mountains (Bauer and Williams, 1989; Williams, 1991).

The Paleoproterozoic Hondo Group (Fig. 3) overlies the Vadito Group and consists of a 1-1.5-km-thick, locally aluminous quartz-ite called the Ortega Formation. Interbedded schists and quartzites of the Rinconada Formation with an apparent maximum thickness of 725 m overlie the Ortega Formation. These sediments were deposited between ca. 1.65 Ga and 1.50 Ga and are interpreted as inner to outer shelf deposits and deltaic deposits, respectively, and may represent a passive margin sequence (Soegaard and Eriksson, 1986; Jones et al., 2009, 2011; Daniel et al., 2013b). The overlying Pilar and Piedra Lumbre Formations were previously included within the Hondo Group by Bauer and Williams (1989). However, based upon recent detrital zircon and metatuff ages from the Piedra Lumbre and Pilar formations, Daniel et al. (2013b) assigned them to the Mesoproterozoic Trampas Group (Fig. 3).

The Pilar Formation is a distinctive, fine-grained, black, carbonaceous phyllite and schist with thin, white schistose layers interpreted as metatuff (Daniel et al., 2013a, 2013b). The base of the formation is marked by a 2-3-m-thick blue-black, garnet-bearing quartzite. The uppermost Pilar Formation shows a gradational transition into the overlying schist and quartzite of the Piedra Lumbre Formation (Bauer, 1988). Centimeter-scale graded beds and centimeter-to-meter scale cross-bedded quartz-ite beds become more abundant and thicker from the lower to upper parts of the exposed Piedra Lumbre section (Bauer, 1988; Long, 1974). The uppermost part of the exposed unit consists of 20-25-m-thick cross-bedded quartzite and black phyllite (Bauer, 1988). The apparent maximum thicknesses of the Pilar and Piedra Lumbre Formations are ~600 m and 400 m, respectively. These formations may represent a retroarc foreland basin although other interpretations are possible (Daniel et al., 2013b).

The Mesoproterozoic Marqueñas Formation (Fig. 3) consists of a basal polymictic boulder to cobble conglomerate overlain by a cross-bedded quartzite. The top of the formation transitions back into a strongly sheared quartz pebble conglomerate. The total apparent thickness is ~500 m, and prior detrital zircon analyses give a maximum depositional age of ca. 1460 Ga (Jones et al., 2011; Daniel et al., 2013b). The Marqueñas-Vadito Group contact is interpreted to represent an unconformity, and the Marqueñas Formation contact with the Pilar and Piedra Lumbre Formations is a ductile shear zone (Bauer, 1993; Daniel et al., 2013a) (Fig. 2). The Marqueñas Formation is interpreted as a synorogenic conglomerate deposit (Jones et al., 2011; Daniel et al., 2013b; Gray et al., 2015).

Spatial Distribution of Aluminum Silicate Polymorphs

Multiple coexisting Al2iO5 polymorphs are commonly associated with aluminous quartzite and schist of the Ortega Formation and more rarely with pelitic schists (Fig. 4). The Ortega Formation is exposed in the Tusas, Picuris, Truchas Peaks, Rio Mora, Rincon, Cimarron and Taos Mountains of northern New Mexico (Figs. 2 and 4). In the Tusas Mountains, metamorphic grade decreases from garnet + biotite + sillimanite schist in the south to pyrophyllite-bearing quartzites in the north. To the east and north, the Rincon, Cimarron, and Taos Mountains expose some of the deepest crustal levels characterized by sillimanite + K-feldspar-bearing rocks and localized evidence for anatexis (Figs. 2 and 4). This distribution of minerals is consistent with a lower geothermal gradient to the north, resulting in similar temperatures at higher pressures (Grambling, 1981) (e.g., Fig. 5).

Al2SiO5 isograd geometry redrawn from Grambling (1981) shows the distribution of polymorphs with respect to elevation along two N-S lines of section in the Truchas Peaks area. Solid gray lines represent isograds for Ky—kyanite; Sil—sillimanite; And—andalusite. Isograds overprint the major regional fold geometry (Grambling, 1981; Grambling et al., 1989). Distribution of Al2SiO5 polymorphs along the line of section represented by single letter abbreviations: A— andalusite; F—fibrolite; K—kyanite; S—sillimanite and multiple letters where they coexist within a sample. Hachured area represents the general area where it is possible to have three coexisting polymorphs. This isograd geometry is a consequence of regional metamorphism generally outlasting deformation (Grambling, 1981; Grambling and Williams, 1985).

Al2SiO5 isograd geometry redrawn from Grambling (1981) shows the distribution of polymorphs with respect to elevation along two N-S lines of section in the Truchas Peaks area. Solid gray lines represent isograds for Ky—kyanite; Sil—sillimanite; And—andalusite. Isograds overprint the major regional fold geometry (Grambling, 1981; Grambling et al., 1989). Distribution of Al2SiO5 polymorphs along the line of section represented by single letter abbreviations: A— andalusite; F—fibrolite; K—kyanite; S—sillimanite and multiple letters where they coexist within a sample. Hachured area represents the general area where it is possible to have three coexisting polymorphs. This isograd geometry is a consequence of regional metamorphism generally outlasting deformation (Grambling, 1981; Grambling and Williams, 1985).

Summary diagram of previous P-T paths proposed for Al2SiO5 triple-point assemblages. (A) Single, counterclockwise metamorphic P-T path for Truchas Peaks and Picuris Mountains (Grambling et al., 1989). K—kyanite; A—andalusite; S—sillimanite. (B) Polymetamorphic P-T paths for the Picuris, Rincon, and northern Taos mountains (modified from Karlstrom et al., 2004). In this model, regional thrusting and metamorphism resulted in the burial of supracrustal rocks and growth of kyanite and sillimanite at ca. 1650 Ma associated with the Mazatzal orogeny. The rocks partially decompressed to mid-crustal depths and remained there for ca. 200 m.y., represented by black P-T loop. Subsequent isobaric heating across the andalusite and silliman-ite stability fields, and the reactivation of preexisting foliation occurred at ca. 1400 Ma, represented by gray P-T loop. (C) Summary diagram of Al2SiO5-quartz O-isotope thermometry modified from Larson and Sharp (2005). Dashed line represents average temperature with 1-σ error indicated by the shaded area for each aluminum silicate-quartz pairing. Polymetamorphic P-T path (after Larson and Sharp, 2005), similar to Fig. 6B, is superimposed to illustrate a possible sequence of growth for the Al2SiO5 polymorphs. Triple points H and P from Holdaway (1971) and Pattison (1992), respectively. ky—kyanite; sil—sillimanite; and—andalusite; f—fibrolite. (D) Clockwise P-T-t-D path for the northern Picuris Mountains from Daniel and Pyle (2006) based upon Al2SiO5 reaction sequence kyanite to sillimanite to andalusite, monazite geo-chronology, and monazite-xenotime geothermometry from the Ortega Formation. See Day 4, Stop 2 for more information. Mzt—monazite.

We define the south-central Tusas Mountains, the Picuris Mountains, Truchas Peaks, and Rio Mora areas as the Al2SiO5 triple-point terrane. In these areas, aluminous quartzite coincides with peak metamorphic conditions that span the Al2SiO5 triple point (Figs. 2 and 4). We attribute early kyanite growth to pyro-phyllite breakdown. Subsequent sillimanite and/or andalusite growth occurs through both direct polymorphic replacement of previous aluminum silicate minerals and Carmichael-type reaction with muscovite replacing the unstable polymorph and the new, stable polymorph replacing muscovite (Carmichael, 1969). Notably, adjacent pelites often contain no, or only one, Al2SiO5 polymorph; more rarely are two aluminum silicates reported (Holdaway, 1978; Grambling, 1981). Grambling et al. (1989) interpreted the occurrence of coexisting Al2SiO5 polymorphs across more than 75,000 km2 as the triple-point terrane. Although the geographic distribution of Al2SiO5 polymorphs is extensive, not all areas contain more than one polymorph, and Al2SiO5-bearing rocks have distinct metamorphic P-T-t history across the terrane (Aronoff et al., 2016). Importantly, the “triple-point” met-amorphic rocks grade into greenschist facies and near-granulite facies rocks that represent shallower and deeper crustal levels, respectively. We wish to emphasize that there are important differences in metamorphic P-T conditions across northern New Mexico and that the entire region did not experience peak meta-morphic temperatures and pressures near the triple point.

Previous Work in the Al2SiO5 Triple-Point Terrane

Equilibrium Interpretations

The coexistence of kyanite, sillimanite, and andalusite in northern New Mexico was first described by Holdaway (1978) for aluminous quartzites in the Picuris Mountains (Figs. 2 and 4). Some of these rocks will be examined on Day 4 of this field trip. He described the occurrence and chemistry of minerals from chloritoid + Al2SiO5-bearing quartzite of the Ortega Formation and the adjacent biotite + staurolite-bearing rocks of the Rinconada Formation. He noted that the chloritoid-bearing assemblages commonly coexisted with one or two or, more rarely, three Al2SiO5 polymorphs. He interpreted a paragenetic sequence of kyanite to andalusite to sillimanite on the basis of mineral textural relationships. Sillimanite contained ~0.65 wt% Fe2O3, but no analyses of kyanite or andalusite were reported (Holdaway, 1978). Holdaway (1978) surmised that the adjacent formations had experienced the same P-T history, and used mineral compositions to estimate the offset of experimental phase equilibria to calculate peak metamorphic conditions of ~530 °C and 3.7 kbar.

The eastern side of the Picuris Mountains is cut by the north-striking, strike-slip Picuris-Pecos fault (Fig. 2). About 28 km south, on the opposite side of the fault are the Truchas Peaks. Similarities in the rock types and correlation of fold axes between the two mountain ranges were recognized by Montgomery (inMiller et al., 1963), who proposed that the rocks exposed in the two mountains ranges had once been adjacent to each other (Fig. 4). Grambling (1981) described the textures and chemistry of the Al2SiO5 polymorphs in the Truchas Peaks area and mapped the Al2SiO5 isograd geometry (Fig. 5). All three polymorphs contain measurable amounts of Fe3+ with up to 0.90 wt% in kyanite and sillimanite and up to 1.7 wt% in andalusite (Grambling, 1981). The Al2SiO5 paragenetic sequence was interpreted as kyanite to andalusite to sillimanite. The Al2SiO5 isograds crosscut major folds and axial planar foliations, and show that peak metamorphic conditions generally outlasted regional deformation involving north-directed shortening (Grambling, 1981; Grambling et al., 1989).

Both Holdaway (1978) and Grambling (1981) interpreted the coexisting aluminum silicate minerals as equilibrium mineral assemblages, with the metastable persistence of kyanite and/ or andalusite, the result of minimal reaction overstepping associated with peak metamorphic conditions near the Holdaway (1971) triple point. Holdaway (1978, p. 1410) commented, “Ideally all rocks should show the sequence kyanite to andalusite to sillimanite, but in some rocks kinetics prevented reaction, and in others the andalusite field was crossed and sillimanite began to grow before andalusite could get started.” Thus, the importance of reaction kinetics was recognized early on as a factor in the distribution of Al2SiO5 polymorphs. The inferred P-T path for these two areas involves isobaric heating to just below the triple point into the sillimanite stability field (Fig. 6A). In support of the Holdaway (1971) triple point, Grambling (1984) reported the occurrence of paragonite + sillimanite + quartz in apparent textural equilibrium from the Truchas Peaks and Rio Mora areas. A detailed description of the spatial distribution and chemistry of the Al2SiO5 polymorphs in the Tusas, Picuris, Truchas Peaks, and Rio Mora areas was presented by Grambling and Williams (1985). They observed significant substitution of Fe3+ and Mn3+ for Al in andalusite and estimated the offset of the andalusite-sillimanite reaction for each area (Fig. 7). Andalusite from the Rio Mora area showed the greatest amount of Fe3+ and Mn3+ substitution and, consequently, the triple point shifts ~40 °C, 0.8 kbar along the kyanite-sillimanite phase boundary. The Truchas Peaks record an offset of ~30 °C, 0.4 kbar, and andalusite compositions from the Picuris Mountains yield minimal offset relative to the triple point (Fig. 7) (Grambling and Williams, 1985).

Offset of the Al2SiO5 triple point due to substitution of Fe3+ and Mn3+ for Al3+ in andalusite from the Picuris, Truchas Peaks, and Rio Mora areas (redrawn from Grambling and Williams, 1985). and—andalusite; ky— kyanite; sil—sillimanite.

Offset of the Al2SiO5 triple point due to substitution of Fe3+ and Mn3+ for Al3+ in andalusite from the Picuris, Truchas Peaks, and Rio Mora areas (redrawn from Grambling and Williams, 1985). and—andalusite; ky— kyanite; sil—sillimanite.

Metamorphic rocks of the Picuris Mountains were sampled for the early, pioneering work in garnet nucleation and growth by Carlson (1989). He examined the spatial distribution, size distribution, and chemical composition of garnet in eleven samples of the Rinconada Formation, near the contact of the R5 quartzite and R6 schist. Statistical measurements indicated a relatively ordered spatial arrangement for nearly all samples. Carlson (1989) proposed a model of thermally accelerated diffusion-controlled growth to explain the observed garnet zoning profiles and size distribution. He concluded that intergranular diffusion was the dominant kinetic control on porphyroblast growth for those samples.

Polymetamorphic-Disequilibrium Interpretations

Polymetamorphic P-T-t-D paths were proposed for multiple areas in northern New Mexico including the Taos and Rincon Mountains and the “triple-point” metamorphic mountain ranges (Pedrick et al., 1998; Read et al., 1999; Williams et al., 1999; Karlstrom et al., 2004; Larson and Sharp, 2005). These paths (Fig. 6B) show a clockwise P-T loop around the Al2SiO5 triple point. Burial and heating at 1.7-1.65 Ga produced kyanite and sillimanite followed by decompression and cooling to an ambient geotherm at midcrustal pressures of ~4 kbar for a 250 Ma resident time. Subsequent isobaric reheating and reactivation of deformational fabrics and preexisting folds at ca. 1.4 Ga was responsible for andalusite and a second generation of sillimanite.

Larson and Sharp (2005) used stable isotope thermometry of coexisting Al2SiO5 polymorphs with quartz to calculate growth temperatures for kyanite, andalusite, sillimanite, and fibrolite from the Truchas Peaks. Their results are presented in Figure 6C and illustrate the crystallization temperatures for the Al2SiO5 polymorphs at disparate temperatures and pressures, inconsistent with the equilibrium occurrence of coexisting polymorphs. The average kyanite-quartz temperature from the Ortega and Rinconada Formations combined was 575 °C ± 20 °C (1σ, n = 15). Prismatic sil-limanite from the Ortega Formation yielded an average temperature of 640 °C ± 15 °C (1σ, n = 3). Fibrolite-quartz temperature estimates from the Rinconada and Ortega Formations yielded two populations with temperatures of 580 °C ± 5 °C (1σ, n = 3) and 700 °C ± 30 °C (1σ, n = 8). Andalusite-quartz oxygen isotope temperature estimates yielded 615 °C ± 20 °C (1σ, n = 6) for the Rinconada Formation. Larson and Sharp (2005) placed their temperature estimates in the context of the polymetamorphic models previously proposed for the region. Figure 6C shows a P-T path involving heating and burial due to inferred contractional deformation resulting in kyanite and sillimanite growth at ca. 1.65 Ga, followed by near isobaric reheating at ca. 1.4 Ga, and growth of andalusite followed by prismatic sillimanite and then fibrolite.

The Past Decade of Research in the Triple-Point Terrane

Following the unpublished M.S. work of Daniel (1992), Daniel and Pyle (2006) presented a new textural interpretation of the Al2SiO5 reaction sequence and also integrated monazite geochronology and monazite-xenotime thermometry to test previously published P-T-t-D paths proposed for the region. They observed both prismatic sillimanite and folded and kinked sil-limanite fibers within relatively undeformed andalusite por-phyroblasts, indicating a paragenetic sequence of kyanite to sillimanite to andalusite. They proposed a clockwise P-T path involving decompression from the sillimanite to andalusite field (Fig. 6D). Monazite grains were observed as inclusions within kyanite grains, sillimanite bundles, and andalusite porphyroblasts and also in the matrix. In situ, electron microprobe chemical ages and ion probe isotopic ages indicate that monazite cores formed at 1436 Ma and rims formed 1400-1380 Ma. Thus, there was no evidence in these samples of Paleoproterozoic amphibolite-facies metamorphism in the Picuris Mountains as proposed by Karlstrom et al. (2004).

Recent work in the region has focused on detrital zircon geochronology, isotopic dating of metavolcanic rocks, and Lu-Hf dating of garnet from multiple structural levels to constrain the timing of deposition, metamorphism, and deformation across the region (Jones et al., 2009, 2011; Daniel et al., 2013b; Aronoff et al., 2016). Detrital zircon and metatuff depositional ages show that the Pilar, Piedra Lumbre, and Marqueñas Formations are deposited between ca. 1490 Ma and 1450 Ma, some 200 m.y. younger than originally proposed for the region (Bauer and Williams, 1989; Jones et al., 2011; Daniel et al., 2013b). Garnet Lu-Hf ages are younger to the north across recognized thrust faults. Aronoff et al. (2016) proposed a model of thrust stacking and thermal equilibration to explain the triple-point metamorphism across the region (Fig. 8). More detailed results from these studies are discussed below in the stop descriptions.

Schematic model for the sequential development of the structure in north-central New Mexico associated with the Picuris orogeny (modified from Aronoff et al., 2016). Mineral abbreviations follow Kretz (1983). and—andalusite; ky—kyanite; sil—sillimanite. (A) Schematic cross section showing the Trampas Group deposited unconformably on the Paleoproterozoic Hondo and Vadito Groups (Daniel et al., 2013a, 2013b). (B-D) Sequential time steps in the tectonic development of the region. Each time step displays the inferred pressure-temperature (P-T) conditions (Hunter, 2013), garnet textures, and schematic cross section for the time interval represented. P on P-T diagram indicates triple point of Pattison (1992), and H the triple point of Holdaway (1971). (B) Inferred P-T conditions for metamorphic rocks between ca. 1460 and 1450 Ma. Garnet sketches from the southern Picuris Mountains (1) and southern Tusas Mountains (2) are labeled with Lu-Hf garnet age; location 2 has not yet started to grow garnet. Cross section shows early thrusting, development of overturned to recumbent folds, deposition of the synorogenic Marqueñas Formation (Jones et al., 2011; Daniel et al., 2013a, 2013b), and the formation of S1 foliation. Depths of sample localities are schematic in cross section because samples are found both along and across strike from each other. (C) Divergent P-T paths for different parts of the terrane are labeled with location numbers 1 through 3. Garnet microstructure sketches show the effects of progressive deformation on garnet from the Picuris Mountains (1) and southern Tusas Mountains (2), as well as the inferred geometry of garnet textures at the time of growth in the central Tusas Mountains (3). Cross section shows upright folding and development of S2 foliation during regional penetrative ductile deformation. Sil-Kfs—sillimanite-K-feldspar. (D) Complete P-T paths followed by samples discussed in this study. Sketches of garnet crystals in their present geometries are organized by metamorphic zone. Cross section shows final phase of north-directed, high-angle thrusting, and development of S3 crenulation cleavage. The final geometry of the Al2SiO5 isograds, crosscutting structures, is shown (Grambling, 1981; Grambling and Williams, 1985; Read et al., 1999). Note that the general geometry of structures in the schematic model panels is similar to previous studies (e.g., Williams, 1991; Williams et al., 1999; Shaw and Karlstrom, 1999), but that the timing of sedimentation, metamorphism, and deformation is different.

Schematic model for the sequential development of the structure in north-central New Mexico associated with the Picuris orogeny (modified from Aronoff et al., 2016). Mineral abbreviations follow Kretz (1983). and—andalusite; ky—kyanite; sil—sillimanite. (A) Schematic cross section showing the Trampas Group deposited unconformably on the Paleoproterozoic Hondo and Vadito Groups (Daniel et al., 2013a, 2013b). (B-D) Sequential time steps in the tectonic development of the region. Each time step displays the inferred pressure-temperature (P-T) conditions (Hunter, 2013), garnet textures, and schematic cross section for the time interval represented. P on P-T diagram indicates triple point of Pattison (1992), and H the triple point of Holdaway (1971). (B) Inferred P-T conditions for metamorphic rocks between ca. 1460 and 1450 Ma. Garnet sketches from the southern Picuris Mountains (1) and southern Tusas Mountains (2) are labeled with Lu-Hf garnet age; location 2 has not yet started to grow garnet. Cross section shows early thrusting, development of overturned to recumbent folds, deposition of the synorogenic Marqueñas Formation (Jones et al., 2011; Daniel et al., 2013a, 2013b), and the formation of S1 foliation. Depths of sample localities are schematic in cross section because samples are found both along and across strike from each other. (C) Divergent P-T paths for different parts of the terrane are labeled with location numbers 1 through 3. Garnet microstructure sketches show the effects of progressive deformation on garnet from the Picuris Mountains (1) and southern Tusas Mountains (2), as well as the inferred geometry of garnet textures at the time of growth in the central Tusas Mountains (3). Cross section shows upright folding and development of S2 foliation during regional penetrative ductile deformation. Sil-Kfs—sillimanite-K-feldspar. (D) Complete P-T paths followed by samples discussed in this study. Sketches of garnet crystals in their present geometries are organized by metamorphic zone. Cross section shows final phase of north-directed, high-angle thrusting, and development of S3 crenulation cleavage. The final geometry of the Al2SiO5 isograds, crosscutting structures, is shown (Grambling, 1981; Grambling and Williams, 1985; Read et al., 1999). Note that the general geometry of structures in the schematic model panels is similar to previous studies (e.g., Williams, 1991; Williams et al., 1999; Shaw and Karlstrom, 1999), but that the timing of sedimentation, metamorphism, and deformation is different.

Field-Trip Descriptions and Stops

Overview

The field stops were selected to show the regional distribution of aluminum silicate minerals in northern New Mexico and to examine both aluminous quartzite and pelitic bulk compositions. One stop will feature coexisting kyanite + sillimanite + andalusite mineral assemblages in aluminous schist, and other stops will emphasize metamorphic mineral assemblages in adjacent formations with varying bulk compositions. Recent mona-zite, zircon, and garnet geochronology results that help to constrain the timing of deposition, metamorphism, and deformation of the region will be discussed at appropriate stops. All mileages for the field stops begin at the parking area of the Sagebrush Inn & Suites, 1508 Paseo Del Pueblo Sur, Taos, New Mexico 87571.

■ Day 1. Southern Picuris Mountains—Vadito Group

Begin at the Sagebrush Inn parking area and drive south on U.S.-68 ~13.8 km (8.6 mi) and look for a pullout on the left (east) side of highway near N 36.3081°, W 105.7308°. Be careful about turning across the oncoming traffic. We will stop here for an introduction to the geology of northern New Mexico. Next, continue south on U.S.-68 for ~20.9 km (about 13 mi) and turn east onto NM-75. Continue east on NM-75 for another 13.7 km (8.5 mi) and park in a wide pullout on the north side of the road near, 36.1979° N, 105.7854° W. We will walk south to 36.1947° N, 105.7854° W to examine rocks at Stop 1. Please watch for fast-moving traffic when crossing the highway.

A simplified geologic map shows the major structures and map units of the Picuris Mountains (Fig. 9) and the approximate locations for stops on Days 1, 2, and 4. Oblique Google Earth image of the southern Picuris Mountains shows stops for Days 1 and 2 and the general geology of the area (Fig. 10).

Simplified geologic map and cross section of the Picuris Mountains showing the major structures including the Copper Hill anticline, Hondo synclinorium, and Plomo-Pecos and Pilar faults (modified from Bauer, 1993; Daniel and Pyle, 2006). Approximate locations for Figures 10 and 21 are shown for the southern and northern Picuris Mountains, respectively.

Simplified geologic map and cross section of the Picuris Mountains showing the major structures including the Copper Hill anticline, Hondo synclinorium, and Plomo-Pecos and Pilar faults (modified from Bauer, 1993; Daniel and Pyle, 2006). Approximate locations for Figures 10 and 21 are shown for the southern and northern Picuris Mountains, respectively.

Spectacular andalusite and andalusite + cordierite schists are exposed at this location (Fig. 10). Porphyroblasts of andalusite range in size from 1-10 cm and are often partially replaced by cordierite. These rocks are representative of the relatively low-pressure, high-temperature P-T conditions of the southern Picuris. Andalusite porphyroblasts located to the northwest at optional Stop 2 are reported to have ca. 1450 Ma monazite inclusions (Williams et al., 1999). These rocks record somewhat higher temperatures and lower pressures than are characteristic of metamorphic rocks in the northern Picuris Mountains. Kya-nite has not been reported in this area to the best of our knowledge, and sillimanite is only reported within the contact aureole of the ca. 1450 Ma Penasco quartz monzonite located a few kilometers to the east (Bauer, 1993; Daniel et al., 2013b).

From Stop 1, drive west on NM-75 about 0.5 km (0.3 mi) and make a slight turn left onto dirt road and continue west about 0.64 km (0.4 mi); park along the roadside near 36.2006° N, 105.7973° W. Walk north along intersecting dirt road and then west into the arroyo near 36.2020° N, 105.7986° W to examine rounded exposures of schist with cm-scale andalusite porphyroblasts. Sigmoidal inclusion trails within andalusite preserve an early northwest-striking, intra-porphyroblast foliation that curves continuously into the northeast-east-striking, south-dipping matrix foliation (Fig. 11A). These rocks were metamorphosed at ~600 °C and 3 kbar (Williams et al., 1999) along a P-T path of isobaric heating and cooling below the Al2SiO5 triple point (Fig. 11B). Figure 12 shows a photomicrograph and compositional zoning maps from garnet in a schist unit ~0.5 km (.3 mi) to the south. These garnet are interpreted as pre-tectonic and gave a Lu-Hf age of 1456 ± 16 Ma (Table 1; Aronoff et al., 2016).

From optional Stop 2, turn around and return to junction with NM-75. At this junction, turn south on another dirt track and follow the signs for the Harding Pegmatite mine about 0.8 km (0.5 mi) and continue to the designated parking area. The Harding Pegmatite is on private property, and permission to visit must be obtained from the Department of Earth and Planetary Sciences, University of New Mexico. As time allows, we will examine metamorphic country rocks of the Vadito Group and the mineralogy of the crosscutting Harding pegmatite. The Harding Pegmatite is a zoned pegmatite with a poorly constrained crystallization age near 1400-1350 Ma. Above the entrance to the mine, the pegmatite clearly intrudes and truncates the dominant regional foliation in the amphibolite country rocks. Kinked and broken spodumene crystals, cuspate-lobate folding along the margin of the pegmatite, and the overall tabular geometry of the pegmatite led Northrop and Mawer (1990) to propose that emplacement of the pegmatite was late syntectonic, during brittle-ductile shearing of the amphibolite host rock. Follow the same hiking route back to the vehicle. Then return to NM 75 along the same route.

From Stop 3, return to NM-75, turn west (left) and drive about 2.4 km (1.5 mi), and then park on the south shoulder of the highway. Carefully cross the road to examine the Vadito schist in the roadcut. These rocks preserve what appear to be altered stau-rolite that rim andalusite porphyroblasts. Matrix monazite near this locality are interpreted to be syn- to post-kinematic and yield ages of 1360-1380 Ma (Stotter, 2016).

■ Day 2. Southern Picuris Mountains—Hondo and Trampas Groups

Depart from the parking lot of the Sagebrush Inn in Taos, New Mexico. Pack your lunch and water and wear good hiking shoes. Drive south on NM 68 ~34.92 km (21.7 mi) and turn south (left) onto NM 75. Continue on NM 75 for another 11.43 km (7.1 mi). The parking area for Stop 1 is on the south side of the road in a large pullout with a single tree, near N 36.2035° and W 105.8067°. Caution is advised as hiking will be off trail, through brush and loose rock, cacti are present, and rattlesnakes may be encountered. Be aware of your surroundings. Review Figure 10 for an overview of stops associated with this day.

Excellent exposures of the basal Marqueñas Formation conglomerate are present on the north side of the road. Please use caution when crossing the highway. We will examine these rocks and begin a traverse through the Mesoproterozoic and Paleoproterozoic section, ending near the hinge of the Copper Hill anticline at the contact between the Rinconada and Ortega Formations. The approximate line of traverse is indicated on Figure 10; labeled field-trip localities correspond to the bulleted descriptions in the following text. At these locations, we will examine the rocks, primary sedimentary features, metamorphic and deformational features, and describe the pertinent geochronological data. Stops 1A-1L may be added or omitted as time allows.

(B) The NM-75 roadcut also exposes the unconformable contact between the base of the ca. 1.45 Ga Marqueñas Formation and the ca. 1.7 Ga Vadito Group (Fig. 13). The top of the Vadito Group is a green-gray micaceous quartzite to quartz-rich chlorite schist with sparse garnet and staurolite and relatively large 200 μηι subhedral to euhedral monazite. Detrital zircons record a unimodal age distribution yielding a peak age of ca. 1.7 Ga (Jones et al., 2011; Daniel et al., 2013b). Matrix monazite from both the Vadito quartzite and Marqueñas Formation yield 207Pb/206Pb ages near 1380-1360 Ma (Stotter, 2016).

Photograph looking west of overturned section with Mesoproterozoic Marqueñas Formation (Ym) overlain by Paleoproterozoic Vadito Group (Xv); Brunton compass for scale, Day 2, Stop 1B. This unconformity (u/c) represents a ca. 260 Ma hiatus based upon the approximate age of the Vadito Group (ca. 1710 Ma) and the Marqueñas Formation (ca. 1450 Ma) (Jones et al., 2011; Daniel et al., 2013a, 2013b). Monazite grains from each side of the unconformity yield similar ages near ca. 1360 Ma (Stotter, 2016).

(C) Quartzite of the middle Marqueñas Formation contains minor amounts of muscovite ± chlorite ± biotite ± epidote ± tourmaline and hematite. Cross-bedding and scours indicate that the Marqueñas Formation is overturned and youngs to the north. Detrital zircon from this horizon yield a minimum peak age of ca. 1471 Ma.

(D) Cobble conglomerate of the upper Marqueñas Formation (Fig. 14) varies from a pebble to cobble to boulder conglomerate. Detrital zircon from this locality yielded the youngest age population in the Picuris Mountains, having a minimum peak age of ca. 1457 Ma (Jones et al., 2011). These clasts are more strongly sheared due to the proximity of the Plomo fault.

(E) The Plomo-Pecos fault separates the overturned north-facing Marqueñas Formation from the upright, south-facing Piedra Lumbre Formation (Bauer, 1993). Conglomerate clasts show increasing strain approaching this contact. Stratigraphic separation is not well constrained across this fault but, as indicated by the juxtaposition of the Marqueñas-Vadito rocks over the Piedra Lumbre Formation, must be a minimum of 3-4 km. In addition, this shear zone postdates regional folding in the Picuris Mountains, as the geometry requires a shear zone cutting across folded strata (Aronoff et al., 2016).

(F) Garnet + biotite ± staurolite schist of the Piedra Lumbre Formation. A north-striking crenulation cleavage (S3) is especially well developed in these schists. This locality was sampled for Lu-Hf garnet geochronology, yielding an isochron age of ca. 1400 ± 3.4 Ma (Aronoff et al., 2016).

(G) The Pilar Formation is a fine-grained, graphitic phyllite with no obvious metamorphic porphyroblasts The Pilar Formation is better exposed in other areas and will be more closely examined at Stop 2.

(H) Contact between the Pilar and Rinconada Formations is coincident with a vitreous, black, garnet-bearing quartzite. This layer is fault repeated at this locality. The layer was initially interpreted as the base of the Pilar Formation by Bauer and Williams (1989), but Daniel et al. (2013b) place it at the top of the Rinconada Formation based upon the similarity in detrital zircon age populations. Detrital zircon from this layer show a broad, unimodal peak with a minimum peak age of ca. 1678 Ma and a dominant peak age of ca. 1759 Ma (Daniel et al., 2013b). Below the quartzite is the uppermost R6 unit of the Rinconada Formation, a biotite + garnet ± staurolite phyllite/schist that locally displays significant transposition of compositional layering. Locally, graded bedding is observed.

(I) Rinconada quartzite (R5) with cross-stratification shows that bedding is upright and youngs to the south, opposite of north-facing cross beds in the Marqueñas Formation. Detrital zircon from one sample collected at this locality produced an age spectrum dominated by a single age probability peak at 1763 Ma and a relatively minor peak at 2592 Ma (Jones et al., 2011).

Photograph looking west of Rinconada schist (R6) with distinct compositional layering folded by F2 parasitic fold along the southern, upright limb of the Copper Hill anticline, Day 2, Stop 1J.

(K) Rinconada schistose unit R4 is exposed in between the R5 and R3 quartzites. This rock is fine- to medium-grained, gray to silver, garnet + staurolite + biotite schist. Quartz-rich layers and biotite-rich layers are common. Garnet crystals tend to be small, euhedral, and concentrated in thin reddish layers. Euhedral staurolite occur throughout the unit.

(L) Uppermost Ortega Formation near the hinge of the Copper Hill anticline. The contact between the lowermost Rinconada (R1/R2) and the underlying Ortega Formation is exposed in the canyon. The lowermost R1 schist is characterized by cm-scale andalusite ± cordierite schist with andalusite becoming less abundant moving up through the unit. The base of the R2 schist is defined by the occurrence of staurolite coexisting with andalusite (Bauer, 1988) and grades into a garnet + staurolite + biotite schist. The R1 and R2 schists are generally combined as a single mappable unit across the Picuris Mountains (Bauer and Helper, 1994). The Ortega Formation quartzite contains kyanite, andalusite, and kyanite + andalusite + chloritoid at this locality (Holdaway, 1978). The Ortega Formation will also be examined further at Stops 1 and 2, Day 4.

Stop 2. Pilar Formation Location: park at 36.2112° N, 105.8273° W

Return to vehicle for a short ride to examine a folded metatuff layer within the Pilar Formation. Stop 2 is located at 36.2123° N, 105.8298° W, but parking for this stop is on the shoulder of the east side of NM 75 at 36.2096° N, 105.8256° W. A trail leads north from the shoulder of the highway into Piedra Lumbre Canyon. This stop will examine the graphitic Pilar Formation. Alternating gray to white schistose layers and black, graphitic phyllitic layers may represent turbidite sequences. The thicker, white metatuff layers are fine grained and consist largely of pla-gioclase + muscovite + phlogopite + K-feldspar with little to no quartz and are folded (Fig. 16). The mineralogy of this sample may in part reflect the mineralogy of the metavolcanic protolith. Zircon recovered from the metatuff layers at this location are small, 20-50 μm in length, and euhedral. They yield crystallization-depositional ages of ca. 1480 Ma (Daniel et al., 2014) and give a key constraint on the timing of deposition for the Trampas Group. Following this stop, return to vehicle for the drive back to the Sagebrush Inn along the same roads used earlier in the day.

Folded metatuff in Pilar formation, looking west, Day 2, Stop 2. There is some debate between the authors about the timing of these folds. Do they represent F2 folds associated with the formation of the Copper Hill anticline, or do they represent F1 folds being refolded around the F2 Copper Hill anticline?

Folded metatuff in Pilar formation, looking west, Day 2, Stop 2. There is some debate between the authors about the timing of these folds. Do they represent F2 folds associated with the formation of the Copper Hill anticline, or do they represent F1 folds being refolded around the F2 Copper Hill anticline?

Today’s stops will focus on rocks in the southern and central Tusas Mountains. The southern Tusas Mountains expose the highest grade rocks we will visit during the trip, with sil-limanite-bearing assemblages developed through garnet and staurolite breakdown reactions. We will then visit spectacular outcrops of kyanite-quartz rocks (>>50% kyanite), which are part of the Vadito Group. We will visit locations described in Bishop (1997), Barnhart et al. (2012), and Aronoff et al. (2016). At Cerro Colorado, we will discuss reaction textures, and compare and contrast alternative models for the P-T path at Cerro Colorado; near isobaric heating (Bishop, 1997), as well as isothermal decompression (Barnhart et al., 2012) have been proposed for these rocks.

(10.6 mi). Turn right onto NM 414 toward Ojo Caliente Mineral Springs Resort and Spa. Continue west-northwest past the parking area and buildings for the spa, and follow NM 414 up the hill behind the spa towards Rincon overlook. Note the road immediately becomes a dirt track and requires a high-clearance vehicle, and should not be attempted during rainy weather. Drive ~3.6 km (1.9 mi) to the Joseph pegmatite mine. Road conditions may make road impassible well before the pegmatite mine, so use caution in driving. Park along the roadside, and don’t block the road as it is used by all-terrain-vehicle, horse, and mountain bike traffic. A simplified geologic map of Cerro Colorado shows the major map units and structure of the area and the locations of Stops 1-4 (Fig. 17).

The Joseph Pegmatite mine is located within the Ojo Caliente No. 1 mining district. The mine contains mica, feldspar, rare earth elements (REE), Ta, Nb, U, Th, and Ba and intrudes the Vadito Group (Mclemore, 2011). The pegmatite is similar to ca. 1400 Ma pegmatite found within the Petaca mining district. The mine primarily produced scrap mica between 1954 and 1965, and was evaluated for Ta and Bi in 2001-2002, but has not been developed (Mclemore, 2011). Mica books and large crystals of quartz and K-feldspar can be easily picked from the floors of the adits; fluorite, garnet, columbite, monazite, and beryl occur as accessory phases.

From the mine, proceed on the dirt track toward the northwest, passing into the arroyo north of the pegmatite mine. Stop 2 is located in outcrops of sillimanite + garnet gneiss and schist of the Vadito Group found within the arroyo.

Stop 2 exposes garnet + sillimanite schist in outcrops along the bottom and sides of the arroyo. These rocks were studied by Bishop (1997), Barnhart et al. (2012) who focused on phase equilibria and metamorphic P-T conditions at Cerro Colorado, and Aronoff et al. (2016) who dated garnet from this locality using Lu-Hf geochronology. Representative mineral textures are shown in Figure 18.

The rocks at this locality commonly contain the mineral assemblage garnet + sillimanite + biotite + muscovite + plagioclase + quartz + ilmenite + tourmaline ± staurolite. Garnet is rimmed by sillimanite and biotite (Fig. 18A). Staurolite, where present, occurs as ragged small crystals and does not appear to be in textural equilibrium with other phases in the rock. Some rocks do not contain muscovite, and K-feldspar has not been observed, suggesting rock bulk composition controls the presence or absence of muscovite, not metamorphic conditions in excess of the stability of muscovite and quartz (Barnhart et al., 2012).

Bishop (1997) provided the first detailed study of metamorphic rocks at Cerro Colorado. An isobaric P-T path was inferred for the Cerro Colorado area based on reaction textures, petro-genetic grids, and thermobarometry (Fig. 19). Key to Bishop’s interpretation is the occurrence of andalusite in Mg-rich schists. Textures in these rocks suggest a prograde sequence from kya-nite to andalusite to sillimanite, which is interpreted to indicate heating at conditions just below the triple point (Fig. 19). Bishop (1997) inferred peak conditions of ~650 °C between 3 and 4 kbar. These rocks occur to the north and east of outcrops visited during this trip (Fig. 19).

Barnhart et al. (2012) focused their analysis on Fe-rich schists, including those found at this outcrop. In contrast to Bishop (1997), Barnhart et al. (2012) favored an interpretation of decompression at high temperature, from conditions in the kyanite stability field into the sillimanite stability field, reaching conditions of 3-4 kbar at ~650 °C (Fig. 19). Barnhart et al. (2012) emphasized the operation of two reactions with shallow but opposite slopes in P-T space to infer decompression: (1) muscovite + garnet = staurolite + biotite, and (2) muscovite+ garnet = sillimanite + biotite. These reactions are separated in P-T space by the reaction (3) staurolite + quartz = garnet + sillimanite, which represents the terminal stability of stauro-lite in Fe-rich compositions. The sillimanite rinds on garnet at this locality are interpreted to result from the operation of reaction 2. While staurolite is present in these rocks, it is generally ragged and does not appear to be part of the peak equilibrium mineral assemblage.

Aronoff et al. (2016) dated garnet from this locality, and it yielded an age of 1400 ± 9 Ma (Table 1). Interpretation of this age is complicated by the effects of garnet dissolution through the operation of reaction 2. Kelly et al. (2011) showed that when garnet is partly dissolved, the preferential loss of Hf and retention of Lu may produce Lu-Hf isochron ages younger than the growth age and older than the time of dissolution. As a result, interpreting garnet age data can be difficult. We will discuss the interpretation of this age in more detail after we have seen rocks that record the operation of reaction 1 in staurolite-bearing rocks.

From Stop 2, return to the dirt track and follow it toward the north-northwest to Stop 3. Note the presence of Tertiary rocks resting in unconformity on the Precambrian rocks along the north side of the road. Many of the schists and gneisses found along the dirt road contain abundant sillimanite knots likely produced by the breakdown of staurolite or garnet.

An unusual feature found here at Cerro Colorado is the occurrence of a kyanite-quartz vein that cuts across the road. The vein also contains muscovite and sillimanite, with silliman-ite clearly postdating the growth of the kyanite (Barnhart et al., 2012). These rocks and others in the area clearly show that iron-rich schist at Cerro Colorado records the transition from kyanite to sillimanite. Andalusite-bearing assemblages have not been observed (Barnhart et al., 2012), contrasting with the presence of andalusite in Mg-rich schist (Bishop, 1997) found 500 m to the northeast (Fig. 17).

Al2SiO5-bearing veins are known at several localities in New Mexico, including the contact aureoles of 1.4 Ga granites in the Manzano and Sandia mountains, in the central mountains (focus of Stop 4 later this afternoon), and the Rincon and Cimarron Mountains. These types of veins are of particular interest because Al2SiO5 is typically less soluble in metamorphic fluids (Yardley, 1986; Kerrick, 1988), leading to the conclusion that metasomatism is the primary process for the formation of Al2SiO5 veins (Kerrick, 1988). Cesare and Hollister (1995) studied andalusite-bearing veins in a contact aureole at Vedrette di Ries, Italy, and concluded that the fluids that deposited the andalusite veins were in equilibrium with host rock fluids, and that the chemicals diffused to the veins where the andalusite nucleated along the walls of hydrofractures. Thus, metasomatism is not required, implying a closed system at the scale of the vein and pelitic host rocks (Cesare and Hollister, 1995).

The fluids associated with the Cerro Colorado quartz-kyanite veins have not been studied, so it is unclear if they originated during metasomatism or in a closed system. Host rocks include Fe-rich schists, which contain abundant staurolite and often contain sillimanite, and rarely kyanite (Bishop, 1997; Barnhart et al., 2012; Aronoff et al., 2016). Notably, host rocks contain tourmaline-quartz layers that also commonly contain sillimanite. The presence of abundant tourmaline implies that the fluid composition, at least locally, would deviate from pure water.

The tourmaline-rich layers are exposed in schist outcrops along both sides of the road, and sillimanite-bearing tourmaline quartz rocks are abundant as float along the road. Many of these layers are isoclinally folded by second-generation folds (F2). These folds are on the northern limb of the Cerro Colorado anticline (F3), which folds F2. In many places, sillimanite is aligned parallel to a mineral lineation within S2 and is crenulated by the third-generation crenulation cleavage (S3). These textural relationships suggest that sillimanite was stable during both F2 and F3 at Cerro Colorado. In many other places, the sillimanite occurs as randomly oriented mats. These mats appear to postdate the major phase of deformation, suggesting the final high-temperature metamorphism postdates deformation (Bishop, 1997; Aronoff et al., 2016). Now proceed up the road toward the south-southwest.

Stop 4. Staurolite-Garnet Schist Location: 36.3348° N, 106.0688° W

Near the end of the road are staurolite-garnet schists studied in detail by Barnhart et al. (2012) and dated by Aronoff et al. (2016). These rocks contain garnet + staurolite + sillimanite + muscovite + biotite + plagioclase + quartz, essentially identical to schist at Stop 2. Representative mineral textures are shown in Figure 18B. The primary differences in these rocks and those at Stop 2 are the textures and abundance of the minerals stauro-lite and sillimanite. Here at Stop 4, staurolite is porphyroblas-tic, forming up to centimeter-sized crystals, whereas sillimanite occurs in small amounts as patches of fibrolite. Additionally, staurolite commonly includes garnet and appears to replace muscovite (Fig. 18B). These textures are interpreted to indicate the operation of reaction 1—muscovite + garnet = biotite + stauro-lite—during decompression (Barnhart et al., 2012; Aronoff et al., 2016). Aronoff et al. (2016) dated garnet from this locality and obtained an age of 1450 ± 6 Ma. Note that this age is surprisingly old compared to the age obtained for the rock at Stop 2, with the same mineral assemblage.

Two models may explain the differences in texture and Lu-Hf age between Stops 2 and 4. Barnhart et al. (2012) suggested that the staurolite schist here at Stop 4 evolved on a decompression P-T path at temperatures lower than those seen at Stop 2. This interpretation is based on the apparent textural equilibrium of staurolite here at Stop 4, as compared to its disequilibrium textures at Stop 2. Barnhart et al. (2102) interpreted the difference in texture to higher temperatures at Stop 2, resulting in the stability field for staurolite being exceeded at Stop 2, whereas here staurolite remained stable. Thus, the difference in age could be the result of distinct P-T paths between the two locations. Alternatively, the differences in ages could be the result of variable amounts of garnet decomposition between the two localities, following the models of Kelly et al. (2011).

Aronoff et al. (2016) tested these alternative models using isochemical phase diagrams for rocks at Stop 2 and Stop 4. The models showed that the metamorphic textures could be matched by either isobaric heating and cooling or decompression. Both paths require heating during development of the sil-limanite rims on garnet at Stop 2 and are consistent with garnet included in staurolite. However, the models predict that while garnet is consumed in both rocks, the differences in bulk composition result in much more garnet consumption in rocks with sillimanite rinds than in rocks with garnet included in staurolite (Aronoff et al., 2016).

In the sillimanite-bearing rocks at Stop 2, garnet is first consumed by a continuous reaction garnet + muscovite = biotite + plagioclase. Only a small amount of staurolite is predicted to be produced in these rocks, and the garnet-consuming continuous reaction becomes garnet + muscovite = staurolite + plagioclase + biotite. Finally, when staurolite is eliminated and sillimanite appears, the garnet-consuming reaction becomes garnet + muscovite = biotite + sillimanite + plagioclase. These model reactions match metamorphic textures and are in good agreement with modal estimates made by point counting for rocks at Stop 2. Over the modeled P-T path, ~80% of the garnet is resorbed.

The modeled phase diagram at Stop 2 contrasts with the model for staurolite-bearing rocks at Stop 4. The two compositions undergo similar reaction histories and mineral assemblages, but the modal evolution is strikingly different for the two rock types. The staurolite-bearing rocks at Stop 4 are more aluminous in composition than those at Stop 2, which results in a smaller amount of garnet. A much smaller amount of garnet is consumed, and most of the garnet consumption results in staurolite production. Plagioclase is also produced and continues to grow after staurolite has stopped growing, which is consistent with inclusions of staurolite in plagioclase within rocks at Stop 4. When staurolite breaks down, a small amount of garnet is produced, and a large amount of sillimanite is predicted. Since the modal abundance of sillimanite is relatively small, the maximum stability of staurolite was not substantially exceeded. The phase diagram for Stop 4 suggests peak metamorphic conditions at temperatures somewhat higher than the Al2SiO5 triple point. The amount of garnet consumed at Stop 4 along this P-T path is only ~20%. The models also indicate rocks at Stop 1 and Stop 3 followed a similar P-T path, with temperature differences limited to 50 °C or less (Aronoff et al., 2016).

The variable resorption of garnet at Stop 2 compared to Stop 4 may also provide a mechanism for explaining the contrasting Lu-Hf ages obtained at these locations. Kelly et al. (2011) showed that reaction of garnet, which partitions Lu compared to Hf, in the presence of a Hf sink such as zircon or ilmenite, produces Lu-Hf garnet ages younger than the growth age, and that the calculated isochron age should approach the age of the dissolution event as more garnet is consumed. The measured age for a resorbed garnet population is a function of the mineral partition coefficients, amount of garnet consumed, and time difference between garnet growth and dissolution. Note that this model is built directly on the garnet growth zoning model of Hollister (1966). Applying the garnet dissolution model to Cerro Colorado, Aronoff et al. (2016) calculated that garnet at both locations grew near ca. 1460 Ma, using the constraint of garnet growth in the Picuris Mountains at 1456 Ma and hornblende cooling ages at Cerro Colorado (Barnhart et al., 2012) (Table 1). The age difference between the two Cerro Colorado samples resulted from ~80% garnet dissolution in the rocks at Stop 2, whereas only ~20% of the garnet was resorbed in rocks found at Stop 4.

Return to vehicle by the same path followed up, and drive back to Ojo Caliente. Our next stop is the Big Rock syncline area in the central Tusas Mountains to observe kyanite-quartz mounds, exposures of feldspathic schist, and the Big Rock conglomerate. Turn left onto U.S.-285 north for 2.74 km (1.7 mi). Then turn left onto NM 111 north for 27.2 km (16.9 mi). Take a sharp right onto Forest Rd 45. Stay on Forest Rd 45 for 5.3 km (3.3 mi) and turn right to continue on Forest Road 45. Proceed ~2.35 km (1.5 mi) and park near the Benjamin tank on the southeast side of the road. The cattle tank may be dry. From the parking area, proceed on foot ~0.61 km (2000 ft) to the east, the location of the kyanite-quartz outcrops.

Stop 5. Kyanite-Quartz Deposits Location: 36.5428° N, 106.0796° W

Kyanite-quartz rocks are well known in the Las Tablas area. Figure 20 shows a simplified geological map of the Big Rock syncline (Simmons et al., 2011), and locations of the quartz-kyanite rocks. The rocks are composed of a mixture of quartz, kyanite, and white mica, commonly reaching 100% kyanite in parts of the outcrop. The kyanite quartz rocks occur within aluminous schist, composed of quartz-white mica-feldspar within a largely metavolcanic section of the Vadito Group. The kyanite knobs occur stratigraphically below a garnet + biotite schist, metarhyolite, and the Big Rock conglomerate. Simmons et al. (2011) studied the petrology of the kyanite-quartz deposits and concluded that they resulted from multiple phases of metasomatism that removed mobile elements including Na, K, Ca, and Fe+2, and concentrated SiO2, and Al2O3 relative to host schist. Oxygen isotope thermometry using texturally distinct kyanite and quartz pairs gives temperatures of 530 °C and 590 °C (Simmons et al., 2011) (Fig. 19).

Simplified geological map of the Big Rock syncline area of the central Tusas Mountains. Location of Stop 5 is shown by the box. Geology is after Koning et al. (2005).

Isochemical phase diagrams were used by Hunter (2013) to investigate P-T conditions for garnet-biotite-staurolite schist collected within 100 m of the kyanite deposits (Fig. 19). Garnet in the sample has normal growth zoning, with high-Mn cores, and low-Mn rims (Hunter, 2013). Garnet contains inclusions of chlorite, epidote, and biotite. Garnet locally has rims of margarite, and biotite is partly replaced by chlorite along cleavage planes (Hunter, 2013). The textures in the sample are consistent with a P-T path within the kyanite stability field (Fig. 19). Peak P-T conditions are inferred to be pressures of 6-8 kbar at temperatures between 500 °C and 550 °C. Margarite rims on garnet, and chlorite after biotite require decompression to pressures between 4 and 5 kbar, at temperatures between 450 °C and 500 °C (Hunter, 2013).

Three samples of garnet-bearing schist were collected from the pelitic schist and dates are presented in Aronoff et al. (2016) (Table 1). Two samples from the same outcrop yielded ages of 1405 ± 4 Ma, and 1409 ± 14 Ma. A third sample yielded an age of 1419 ± 19 Ma. All three samples are consistent with garnet growth between ca. 1420 and ca. 1400 Ma, consistent with the onset of prograde metamorphism in the kyanite grade rocks being ca. 30 m.y. younger than metamorphism in the southern Picuris Mountains and at Cerro Colorado. Aronoff et al. (2016) interpreted the time-transgressive growth of garnet to indicate sequential metamorphism of in-sequence thrust-bounded blocks in a north-directed contractional orogen.

If time permits, we will visit outcrops of the Big Rock conglomerate before returning to vehicles. The Big Rock conglomerate occurs in the middle Vadito stratigraphic section. It and adjacent rocks represent clastic sedimentation within a largely volcanic section. Previously, the Paleoproterozoic Big Rock conglomerate was correlated with the Mesoproterozoic Marqueñas Formation of the Picuris Mountains, but differences in the detrital zircon populations show that the two units are distinct. Outcrops of the conglomerate contain many primary sedimentary structures including cross bedding, graded beds, and bedding cleavage relationships that are consistent with the inferred stratigraphy.

Return to the vehicle and drive back on Forest Road 45 to NM 111. Proceed south for 23.5 km (14.6 mi) back to U.S.-285. Turn left onto U.S.-285 N, following the route back to the Sagebrush Inn that was used this morning to reach Ojo Caliente.

Depart the Sagebrush Inn at 7:30 a.m. Drive south on NM 68 ~14.81 km (9.1 mi) and as the highway curves to the west, turn left (south) onto the dirt road. Beware of fast-moving traffic along this bend in NM 68. Drive ~0.64 km (0.4 mi) along this dirt road (Forest Service Road 606) and park in an open area near N 36.2999°, W 105.7319°, and do not block the road. We will hike to Stops 1 and 2 and when we return to the vehicles, we will depart for Denver.

Walk east along the dirt road ~0.64 km (0.4 mi) to examine the Ortega quartzite exposed on the northern limb of the Hondo synclinorium (Figs. 9 and 21). Reddish aluminous layers reveal kyanite and sillimanite aligned in a down-dip mineral lineation (Fig. 22). This outcrop is typical of aluminous layers within the Ortega Formation. Walk farther east, up the drainage, to examine cross-bedded quartzite along the south side of the canyon. The Pilar and Piedra Lumbre Formations are exposed in the core of this fold and preserve a more complete section than is present in the Copper Hill area.

Photomicrographs of samples CD94-29 and CD00-3c from the schistose layer within the Ortega Formation, Day 4, Stop 2. These samples are described by Daniel and Pyle (2006). (A) Coexisting kyanite + sillimanite included within andalusite. (B) Crenulated fibrolite in andalusite porphyroblast that pseudomorphs muscovite. These textures show a transition of kyanite to sillimanite to andalusite. And—andalusite; Ky—kyanite; Sil—sillimanite; St—staurolite; Ms— muscovite; Qtz—quartz.

Photomicrographs of samples CD94-29 and CD00-3c from the schistose layer within the Ortega Formation, Day 4, Stop 2. These samples are described by Daniel and Pyle (2006). (A) Coexisting kyanite + sillimanite included within andalusite. (B) Crenulated fibrolite in andalusite porphyroblast that pseudomorphs muscovite. These textures show a transition of kyanite to sillimanite to andalusite. And—andalusite; Ky—kyanite; Sil—sillimanite; St—staurolite; Ms— muscovite; Qtz—quartz.

Stop 2 is approximately 3.2 km (2 mi) from Stop 1, with a 300 m climb in elevation (Fig. 21). There is no trail and the hike is moderately strenuous across steep, brushy slopes and loose rock. Samples may be collected from this locality, but please be respectful of the outcrops as this is the best locality that we know of for this spectacular mineral assemblage. Figure 21 is a simplified geologic map of the area showing the distribution of Al2SiO5 polymorphs from Holdaway and Goodge (1990) and Daniel and Pyle (2006) and the location of Stop 2. The rocks of Hondo Canyon have been the subject of several studies including Hold-away (1978), Holdaway and Goodge (1990), Goodge and Holdaway (1995), and Daniel and Pyle (2006). The quartzite exposed along the ridge crest contains kyanite + sillimanite + chloritoid ± staurolite and less commonly, andalusite. The schistose layer at Stop 2 contains cm-scale andalusite with inclusions of kyanite, prismatic sillimanite, and fibrolite (Fig. 23A). Fibrolite bundles in the matrix and within andalusite are folded and kinked in a crenulation, and andalusite overgrows the crenulation (Fig. 23B). This key observation led to the clockwise loop around the triple point (Fig. 6D) proposed by Daniel and Pyle (2006). Chloritoid + chlorite + Zn-staurolite also occur in this rock. The oxide is titanhematite (Hem91-94, Ilm9-6) and andalusite contains up to 1.67 wt% Fe2O3, but no detectable Mn3+. The Fe3+ concentration of andalusite is noticeably higher than andalusite compositions previously reported for the Picuris Mountains by Grambling and Williams (1985). Monazites in these rocks are strongly zoned in Y, U, and Th (Fig. 24), and were analyzed by EMP for U-Th-Pb chemical ages and by ion microprobe for U-Pb isotopic ages of 1436 Ma to 1380 Ma. Return to the vehicles before noon and depart for Colorado.

Monazite chemical maps of Y, Th, U, and either Ce or Pb from Ortega Formation, Day 4, Stops 1 and 2. (A) Matrix monazite from quartzite with deep embayments consistent with partial dissolution; (B) monazite inclusion within kyanite; note the relatively euhedral shape and lack of rim zoning compared to the matrix grain; (C) monazite inclusion within andalusite; monazite show three distinct compositional zones with core ages near 1436 Ma and rim ages near 1380 Ma (Daniel and Pyle, 2006).

Discussion

We interpret the coexistence of two and three Al2SiO5 polymorphs in aluminous bulk compositions of the Ortega Formation to represent a single regional metamorphic cycle characterized by nested P-T loops with clockwise paths that experienced peak metamorphic temperatures above and below the triple point. This resulted in the Al2SiO5 reaction sequences of kyanite to silliman-ite, kyanite to sillimanite to andalusite, and kyanite to andalusite, respectively, for the aluminous compositions. Direct polymorphic replacement is observed in the incomplete reaction textures. Andalusite growth is also attributed to a cyclic reaction with andalusite pseudomorphing muscovite and muscovite replacing earlier kyanite and/or sillimanite (Carmichael, 1969; Daniel and Pyle, 2006). The incomplete reactions between the Al2SiO5 polymorphs suggest only minor reaction overstepping, and the Al2SiO5 isograd geometry and mineral chemistry of coexisting polymorphs (Grambling and Williams, 1985) are consistent with a reasonable approach to equilibrium at the regional scale. Garnet growth ages are between ca. 1460 Ma and 1400 Ma, and mona-zite included within kyanite, sillimanite, and andalusite and in the matrix give ages between ca. 1440 Ma and 1380 Ma. There is no evidence for multiple metamorphic cycles, with regional amphib-olite to near granulite facies at ca. 1650 Ma, as proposed by Read et al. (1999), Williams et al. (1999), Karlstrom et al. (2004), and Larson and Sharp (2005). Extant geochronological data instead suggest that metamorphism at ca. 1650 Ma was greenschist facies or less (Figs. 2 and 4). Ongoing work is focused on identifying adjacent regions of New Mexico and Colorado that were and were not affected by the Picuris orogeny. Contact and regional metamorphism associated with ca. 1.65 Ga granitic plutons and tectonism may also be important, but does not appear to be the dominate feature of the “triple terrane.” Continued focus on dating of rock-forming minerals like garnet, accessory phases like monazite, and detrital zircon are clearly needed in regions where traditional U/Pb zircon dating of cross-cutting relationships was the only tool used to date metamorphism and tectonism. We attribute regional triple-point metamorphism and associated contractional deformation to the ca. 1450-1360 Ma Picuris orogeny in northern New Mexico (Daniel et al., 2013b; Aronoff et al., 2016).

The contributions of Lincoln Hollister to petrology are well illustrated by the evolution of thought within the triple-point ter-rane of northern New Mexico. To date, Hollister and three generations of students have been working on the triple-point terrane. Hollister’s work on mineral zoning, triple-point metamorphism, and the relationships between deformation and metamorphism have strongly influenced the evolution of thought within this classic triple-point terrane.

Acknowledgments

Many people have contributed to the understanding of the geology of the “triple-point” terrane in New Mexico. We have tried to highlight the work of as many people as possible, and apologize to anyone we may have missed. We thank reviewers Jamey Jones, David Pattison, and Frank Spear and field trip co-chairs Stephen Keller and Matthew Morgan for their helpful and constructive comments. Daniel’s work in the triple-point terrane was supported by National Science Foundation grants EAR-9909457 and EAR-1250220, and Bucknell University. Andronicos’ work was supported by National Science Foundation grant EAR-1353921 and Purdue University, and Aronoff acknowledges support from ExxonMobil, the Geological Society of America, and Purdue University.

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Simplified lithostratigraphic section for the Tusas and Picuris Mountains. The ca. 1450 Ma Marqueñas Formation is only exposed south of the Plomo-Pecos fault and unconformably overlies the ca. 1710 Ma Vadito Group schist. This section is overturned in the southern Picuris Mountains. The ca. 1488 Ma or younger Trampas Group is exposed north of the Plomo-Pecos fault and continues along strike to the east in the Truchas Peaks and Rio Mora areas. The Rinconada Formation and overlying Trampas Group are not observed in the Tusas Mountains (Bauer and Williams, 1989; Williams, 1991).

Simplified lithostratigraphic section for the Tusas and Picuris Mountains. The ca. 1450 Ma Marqueñas Formation is only exposed south of the Plomo-Pecos fault and unconformably overlies the ca. 1710 Ma Vadito Group schist. This section is overturned in the southern Picuris Mountains. The ca. 1488 Ma or younger Trampas Group is exposed north of the Plomo-Pecos fault and continues along strike to the east in the Truchas Peaks and Rio Mora areas. The Rinconada Formation and overlying Trampas Group are not observed in the Tusas Mountains (Bauer and Williams, 1989; Williams, 1991).

Al2SiO5 isograd geometry redrawn from Grambling (1981) shows the distribution of polymorphs with respect to elevation along two N-S lines of section in the Truchas Peaks area. Solid gray lines represent isograds for Ky—kyanite; Sil—sillimanite; And—andalusite. Isograds overprint the major regional fold geometry (Grambling, 1981; Grambling et al., 1989). Distribution of Al2SiO5 polymorphs along the line of section represented by single letter abbreviations: A— andalusite; F—fibrolite; K—kyanite; S—sillimanite and multiple letters where they coexist within a sample. Hachured area represents the general area where it is possible to have three coexisting polymorphs. This isograd geometry is a consequence of regional metamorphism generally outlasting deformation (Grambling, 1981; Grambling and Williams, 1985).

Al2SiO5 isograd geometry redrawn from Grambling (1981) shows the distribution of polymorphs with respect to elevation along two N-S lines of section in the Truchas Peaks area. Solid gray lines represent isograds for Ky—kyanite; Sil—sillimanite; And—andalusite. Isograds overprint the major regional fold geometry (Grambling, 1981; Grambling et al., 1989). Distribution of Al2SiO5 polymorphs along the line of section represented by single letter abbreviations: A— andalusite; F—fibrolite; K—kyanite; S—sillimanite and multiple letters where they coexist within a sample. Hachured area represents the general area where it is possible to have three coexisting polymorphs. This isograd geometry is a consequence of regional metamorphism generally outlasting deformation (Grambling, 1981; Grambling and Williams, 1985).

Offset of the Al2SiO5 triple point due to substitution of Fe3+ and Mn3+ for Al3+ in andalusite from the Picuris, Truchas Peaks, and Rio Mora areas (redrawn from Grambling and Williams, 1985). and—andalusite; ky— kyanite; sil—sillimanite.

Offset of the Al2SiO5 triple point due to substitution of Fe3+ and Mn3+ for Al3+ in andalusite from the Picuris, Truchas Peaks, and Rio Mora areas (redrawn from Grambling and Williams, 1985). and—andalusite; ky— kyanite; sil—sillimanite.

Schematic model for the sequential development of the structure in north-central New Mexico associated with the Picuris orogeny (modified from Aronoff et al., 2016). Mineral abbreviations follow Kretz (1983). and—andalusite; ky—kyanite; sil—sillimanite. (A) Schematic cross section showing the Trampas Group deposited unconformably on the Paleoproterozoic Hondo and Vadito Groups (Daniel et al., 2013a, 2013b). (B-D) Sequential time steps in the tectonic development of the region. Each time step displays the inferred pressure-temperature (P-T) conditions (Hunter, 2013), garnet textures, and schematic cross section for the time interval represented. P on P-T diagram indicates triple point of Pattison (1992), and H the triple point of Holdaway (1971). (B) Inferred P-T conditions for metamorphic rocks between ca. 1460 and 1450 Ma. Garnet sketches from the southern Picuris Mountains (1) and southern Tusas Mountains (2) are labeled with Lu-Hf garnet age; location 2 has not yet started to grow garnet. Cross section shows early thrusting, development of overturned to recumbent folds, deposition of the synorogenic Marqueñas Formation (Jones et al., 2011; Daniel et al., 2013a, 2013b), and the formation of S1 foliation. Depths of sample localities are schematic in cross section because samples are found both along and across strike from each other. (C) Divergent P-T paths for different parts of the terrane are labeled with location numbers 1 through 3. Garnet microstructure sketches show the effects of progressive deformation on garnet from the Picuris Mountains (1) and southern Tusas Mountains (2), as well as the inferred geometry of garnet textures at the time of growth in the central Tusas Mountains (3). Cross section shows upright folding and development of S2 foliation during regional penetrative ductile deformation. Sil-Kfs—sillimanite-K-feldspar. (D) Complete P-T paths followed by samples discussed in this study. Sketches of garnet crystals in their present geometries are organized by metamorphic zone. Cross section shows final phase of north-directed, high-angle thrusting, and development of S3 crenulation cleavage. The final geometry of the Al2SiO5 isograds, crosscutting structures, is shown (Grambling, 1981; Grambling and Williams, 1985; Read et al., 1999). Note that the general geometry of structures in the schematic model panels is similar to previous studies (e.g., Williams, 1991; Williams et al., 1999; Shaw and Karlstrom, 1999), but that the timing of sedimentation, metamorphism, and deformation is different.

Schematic model for the sequential development of the structure in north-central New Mexico associated with the Picuris orogeny (modified from Aronoff et al., 2016). Mineral abbreviations follow Kretz (1983). and—andalusite; ky—kyanite; sil—sillimanite. (A) Schematic cross section showing the Trampas Group deposited unconformably on the Paleoproterozoic Hondo and Vadito Groups (Daniel et al., 2013a, 2013b). (B-D) Sequential time steps in the tectonic development of the region. Each time step displays the inferred pressure-temperature (P-T) conditions (Hunter, 2013), garnet textures, and schematic cross section for the time interval represented. P on P-T diagram indicates triple point of Pattison (1992), and H the triple point of Holdaway (1971). (B) Inferred P-T conditions for metamorphic rocks between ca. 1460 and 1450 Ma. Garnet sketches from the southern Picuris Mountains (1) and southern Tusas Mountains (2) are labeled with Lu-Hf garnet age; location 2 has not yet started to grow garnet. Cross section shows early thrusting, development of overturned to recumbent folds, deposition of the synorogenic Marqueñas Formation (Jones et al., 2011; Daniel et al., 2013a, 2013b), and the formation of S1 foliation. Depths of sample localities are schematic in cross section because samples are found both along and across strike from each other. (C) Divergent P-T paths for different parts of the terrane are labeled with location numbers 1 through 3. Garnet microstructure sketches show the effects of progressive deformation on garnet from the Picuris Mountains (1) and southern Tusas Mountains (2), as well as the inferred geometry of garnet textures at the time of growth in the central Tusas Mountains (3). Cross section shows upright folding and development of S2 foliation during regional penetrative ductile deformation. Sil-Kfs—sillimanite-K-feldspar. (D) Complete P-T paths followed by samples discussed in this study. Sketches of garnet crystals in their present geometries are organized by metamorphic zone. Cross section shows final phase of north-directed, high-angle thrusting, and development of S3 crenulation cleavage. The final geometry of the Al2SiO5 isograds, crosscutting structures, is shown (Grambling, 1981; Grambling and Williams, 1985; Read et al., 1999). Note that the general geometry of structures in the schematic model panels is similar to previous studies (e.g., Williams, 1991; Williams et al., 1999; Shaw and Karlstrom, 1999), but that the timing of sedimentation, metamorphism, and deformation is different.

Simplified geologic map and cross section of the Picuris Mountains showing the major structures including the Copper Hill anticline, Hondo synclinorium, and Plomo-Pecos and Pilar faults (modified from Bauer, 1993; Daniel and Pyle, 2006). Approximate locations for Figures 10 and 21 are shown for the southern and northern Picuris Mountains, respectively.

Simplified geologic map and cross section of the Picuris Mountains showing the major structures including the Copper Hill anticline, Hondo synclinorium, and Plomo-Pecos and Pilar faults (modified from Bauer, 1993; Daniel and Pyle, 2006). Approximate locations for Figures 10 and 21 are shown for the southern and northern Picuris Mountains, respectively.

Folded metatuff in Pilar formation, looking west, Day 2, Stop 2. There is some debate between the authors about the timing of these folds. Do they represent F2 folds associated with the formation of the Copper Hill anticline, or do they represent F1 folds being refolded around the F2 Copper Hill anticline?

Folded metatuff in Pilar formation, looking west, Day 2, Stop 2. There is some debate between the authors about the timing of these folds. Do they represent F2 folds associated with the formation of the Copper Hill anticline, or do they represent F1 folds being refolded around the F2 Copper Hill anticline?

Photomicrographs of samples CD94-29 and CD00-3c from the schistose layer within the Ortega Formation, Day 4, Stop 2. These samples are described by Daniel and Pyle (2006). (A) Coexisting kyanite + sillimanite included within andalusite. (B) Crenulated fibrolite in andalusite porphyroblast that pseudomorphs muscovite. These textures show a transition of kyanite to sillimanite to andalusite. And—andalusite; Ky—kyanite; Sil—sillimanite; St—staurolite; Ms— muscovite; Qtz—quartz.

Photomicrographs of samples CD94-29 and CD00-3c from the schistose layer within the Ortega Formation, Day 4, Stop 2. These samples are described by Daniel and Pyle (2006). (A) Coexisting kyanite + sillimanite included within andalusite. (B) Crenulated fibrolite in andalusite porphyroblast that pseudomorphs muscovite. These textures show a transition of kyanite to sillimanite to andalusite. And—andalusite; Ky—kyanite; Sil—sillimanite; St—staurolite; Ms— muscovite; Qtz—quartz.

Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.

, Detrital zircon and metatuff age constraints on the timing of Mesoproterozoic deposition and orogenesis ca. 1.50-1.40 Ga across the southwestern United States and possible connections with the geologic evolution of the Belt Supergroup: Geological Society of America Abstracts with Programs,

v

.

46

, no.

6

, p.

827

.

Goodge

,

J.

Holdaway

,

M.J.

,

1995

, Rock-buffered fluid evolution of metapelites and quartzites in the Picuris Range, northern New Mexico: Journal of Petrology, v.

, Proterozoic assembly of North America: Insight from the Interactions of deformation, metamorphism, and magmatism in the southwestern United States [Ph.D. thesis]: Ithaca, New York, Cornell University,